The present invention relates generally to a method for fabricating semiconductor devices and, more particularly, for the selective deposition of a conductor onto a substrate for forming integrated semiconductor circuits.
A variety of fabrication techniques have been developed for deposition of conductors onto substrates to form electrodes, wiring and the like in integrated semiconductor circuits. Because of the compatibility of refractory metals and silicon substrates in integrated circuits, chemical vapor deposition (CVD) of refractory metals and their silicides onto silicon substrates has enjoyed broad interest within the microelectronics industry. However, such interest has been constrained in commercial applications because of limitations in the CVD process as presently practiced.
Selective CVD processes were developed for the deposition of conductors onto a semiconductor substrate as discussed by R. Blower, "Advanced Microelectronics Applications" Solid State Technology, pp.117-126, November 1986. More refined CVD processes have been developed by A. Reisman et al, "Selective Tungsten on Silicon by the Alternating Cyclic, AC, Hydrogen Reduction of WF6" J. Electrochem. Soc. Vol. 137, pp. 772-727, February 1990 as well as; Shioya et al U.S. Pat. No. 4,804,560 and Iyer et al U.S. Pat. No. 4,617,087.
Typically, selective deposition processes employ chemical vapor deposition to selectively deposit a conductor onto a substrate of an integrated semiconductor circuit. For example, the surface of a silicon semiconductor can be coated with a mask layer having apertures patterned therein so as to form openings in which a refractory metal can be deposited. The silicon semiconductor with mask layer can then be loaded into CVD apparatus. A conductor source gas, such as WF6, and H2 are fed into the CVD apparatus; the gas pressure is reduced to less than 1 Torr; the semiconductor substrate is heated in the range of 300.degree. to 600.degree. C.; and chemical reactions occur generally in accordance with the following relationships: EQU 2 WF6+3 Si=2W+3SiF4 (1) EQU WF6+3H2=W+6HF (2)
The mask layer is non reactive in reaction (1) and is typically a material such as SiO2 or phosphosilicate glass (PSG). As such, reaction (1) can be thought of as selectively depositing tungsten only on the exposed silicon substrate. However, reaction (1) is self-limiting once a deposition thickness of .about.100 Angstroms has been deposited onto the silicon substrate. Thereafter, reaction (2) continues to deposit tungsten on the previously deposited tungsten. Since hydrogen dissociates preferentially on metals and silicides and not on SiO2, reaction (2) tends to be selective and can achieve tungsten layers of 500 to 1500 Angstroms thick before the onset of deposition of tungsten on the mask layer itself. At which point, the process ceases to be selective and tungsten is now deposited onto both the mask layer as well as the previously deposited tungsten. This loss of selectivity has been a major constraint in the commercialization of such processes and has been the focus of the efforts previously described by Reisman, Iyer and Shioya. Reisman, Iyer and Shioya each provide different approaches to removing the unwanted deposition of tungsten from the mask layer. In particular, Shioya employs either gaseous or liquid HF as the etchant; Iyer employs a plasma etching procedure using NF3 as the etchant, which occurs simultaneously with the deposition process; and Reisman uses the conductor source gas itself, WF6, as the etchant. Nevertheless, the need for a commercial CVD selective deposition process remains largely unsatisfied.